1. Field
This patent specification relates to analyzing survey data from naturally occurring electromagnetic fields. More particularly, this patent specification relates to methods and systems for analyzing survey data from naturally occurring electromagnetic fields obtained using at least one borehole-deployed receiver.
2. Background
The use of surface to borehole electromagnetic methods has a long history in the mining industry. In general the technique uses a man-made source at the surface of the earth, with measurements made of various electromagnetic field components down-hole. Compared to surface only methods, surface-to-borehole techniques offer improved resolution at depth in the region of the borehole. Dyck, A, 1991, Drill-Hole, Electromagnetic methods p 881-931, in Electromagnetic methods in Applied geophysics edited by M. N. Nabighian, discusses a comprehensive history (mostly for mineral exploration) up until 1991. The use of a purely galvanic-electrical measurement configuration for monitoring saline water injection into an aquifer is provided in Bevc, D. and Morrison, H. F., 1991, Borehole-to-surface electrical resistivity monitoring of a salt-water injection experiment; Geophysics, 56, 769-777. Tseng, H.-W., Becker, A., Wilt, M. J., and Deszcz-Pan, M., 1998, A borehole-to-surface electromagnetic survey; Geophysics, 63, 1565-1572, documents the use of a down-hole magnetic-inductive source with measurements of the magnetic field made on the earth's surface to monitor the same injection process. Spitzer, K, 1983, Observations of geomagnetic pulsations and variations with a new borehole magnetometer down to depths of 300 m; Geophys. J. Int., 115, 839-848, and Jones, A. G., 1983, A passive, natural-source, twin-purpose borehole technique: vertical gradient magnetometry; J. of Geomag. and Geoelect., 35, 473-490, discuss methods with which to measure the magnetotelluric fields down hole. More recently, Scholl, C., and Edwards, R. N., 2007, Marine downhole to seafloor dipole-dipole electromagnetic methods and the resolution of resistive targets; Geophysics, 72, WA39-WA49, discusses a theoretical modeling study to propose using a downhole vertical electric transmitter with measurements of the electromagnetic field made at the seafloor for oil-reservoir exploration and characterization in a marine environment.
Generating a signal at the surface that can be detected downhole can be problematic from a logistics standpoint. If a large loop source is used on the earth's surface, a high enough frequency must be employed such that inductive scattering in the region around the borehole is significant enough to be detected. This can limit the depth to which the method can be employed due to attenuation losses at higher frequencies. A grounded source can be employed at any frequency. However, this requires making good electrical contact (that is, low contact resistance) with the ground, which again can cause problems logistically. In addition, both of these source configurations require the use of a high-power transmitter, which can be problematic to transport in difficult terrain.
WO 2005/085909 discusses making electromagnetic measurements on the surface and also measurements by a sensor in proximity to a reservoir in a wellbore. Initial measurements are used to construct an initial Earth model, and then repeat measurements made at different times during reservoir production to determine the spatial distribution of the fluid contact. The surface measurements can be magnetotelluric measurements. However, there is no discussion of downhole magnetotelluric measurements.
U.S. Pat. No. 4,349,781 discusses a method for measuring the natural fields downhole using a superconducting magnetic field measurement device. However, there is no discussion of using other types of downhole measurement devices and there is no discussion of combining the superconducting magnetic field measurements made downhole with surface magnetotelluric measurements.